Waveguide filters

ABSTRACT

A microwave reflection filter having two waveguide channels each with a narrowing taper between the input and output ports down to dimensions which correspond with a predetermined upper cut-off frequency. The input taper of the first channel has the same dimensions as the output taper of the second channel and the input taper of the second channel is dimensioned so that the distance from the input port to the point of cut-off for any frequency in the reflected band is lambda /4 greater than the distance from the input port of the first channel to the corresponding cut-off point in the first channel.

1111 ttes 1111 3,851,282 atsgn NOV. 26, 1974 WAVEGUIDE FILTERS 2,972,722 2/1961 Ohm 333/34 x 3,611,213 10/1971 Craven etal [75] Inventor: Barry Kenneth Wuhan" 3,634,788 1/1972 Craven 333/73 w I England [73] Assignee: The Marconi Company Limited, Primary Examiner-James W. Lawrence Chelmsford, Essex, England Assistant Examiner-Marvin Nussbaum [22] Filed Nov 2 1973 Attorney, Agent, or Firm-Baldwin, Wight & Brown [2]] App]. No.: 412,504 7 [57] ABSTRACT A microwave reflection filter having two waveguide {30] Foreign Application Priority Data channels each with a narrowing taper between the Nov. 11, 1972 Great Britain 52189/72 input and Output Ports down dimensions which respond with a' predetermined upper cut-off fre- 52 us. (:1. 333/73 w, 333/1, 333/9, q y- The input taper 9f the first channel has the 333 34 333 5 same dimensions as the output taper of the second 51 Im. 01. H0lp 5/12, HOlp 1/20 Cheme! and the input taper 9f the Second Channel is [53] Fi l f S h 333/1 6 9 21 R, 2 A dimensioned so that the distance from the input port 333 33 34 35 73 w to the point of cut-off for any frequency in the reflected band is M4 greater than the distance from the [5 References Cited input port of the first channel to the corresponding UNITED STATES PATENTS cut-off point in the first channel.

2.972.721 2/1961 Ohm 333/34 x 8 Claims, 2 Drawing Figures WAVEGUIDE FILTERS This invention relates to waveguide filters and more specifically to reflection filters in which microwave energy above a predetermined frequency is propagated substantially unaffected through the filter and energy below said predetermined frequency is reflected by the filter.

For use in a band branching network with energy that is split into two propagating channels each of which is capable of supporting energy in the TEE mode in semicircularly sectioned waveguide, a reflection filter known to the applicants, comprises two corresponding separate channels each with an input and an output port and a taper, the tapers in each having the same shape and dimensions and therefore similarly decreasing the cut-off frequency in each of the channels. The tapers reverse after the minimum desired cut-off frequency has been achieved such that the widest apertures are at the input and output ports. in this known reflection filter the tapers are each dimensioned in accordance with a c05 law. However the taper at the input port of one of the channels commences a distance, equal to a quarter waveguide wavelength at the centre frequency of the band of frequencies to be reflected, after the commencement of the taper from the input port of the other channel, with the converse occurring at the output ports.

In operation of the brand branching network, an incident TE mode wave originating in circular waveguide is divided substantially equally into the two propagating channels which each have a semicircular cross section and which are coupled to the reflection filter. TE? mode waves thus enter the input ports of the reflection filter with waves having a higher frequency than the cut-off frequency determined by the narrow part of the taper passing substantially unaffected therethrough and waves below the cut-off frequency being reflected by the taper. When the TE mode wave is divided into two TEfi mode waves, the axial magnetic fields of theTEfi mode waves are 1r radians out of phase with one another. However, because the tapers in the inputs of the channels of the reflection filter are displaced by a quarter waveguide wavelength of the centre frequency of the band of frequencies to be reflected with respect to one another, signals at said centre frequency will be reflected in both channels and will return to the input ports with a difference in phase of 1r radians resulting from their different path lengths. Therefore because of the initial 1r radians phase difference reflected waves at said centre frequency in the two channels are phase matched when'emerging from the input ports of the reflection filter.

Such a known reflection filter however suffers from the disadvantage that only waves of the reflected centre frequency are phase matched so that the reflection filter is suitable only for very narrow band operation.

The present invention seeks to provide a reflection filter that is capable of substantially phase matching TE mode waves over a broad frequency band that is a band of frequencies extending over a range of say 2 GHz or more for centre frequencies in the region of 90 GHz.

According to this invention a microwave reflection filter includes two waveguide channels, both having input and output ports with tapers between said ports such that the channel dimension reduces from said input ports to a region between said ports, down to dimensions corresponding to a predetermined upper cutoff frequency such that signals with frequencies above said cut-off frequency pass through the output ports and signals with frequencies below said upper cut-off frequency are reflected through the input ports, the taper from the input port of one of said channels being dimensioned in accordance with a predetermined law and the taper from the input port of said other channel being dimensioned such that the distance from the input port to the point of cut-off for any frequency in the reflected band is substantially a quarter waveguide wavelength, at the frequency, greater than the distance from the input port of said one channel to the corre sponding cut-off point in said one channel. It is envisaged that in most practical embodiments the waveguide channels will include tapers between said region and the output ports, the taper in said one channel being a mirror image of the taper in the input of said other channel and vice versa, whereby the signals which are at frequencies above said upper cut-off frequency are substantially unaffected by their passage through the filter.

In producing such a filter in accordance with the invention said predetermined law taper may be of any desired suitable form and the shape of the other taper is obtained by calculating said distances for a number of discrete frequencies and interpolating the shape of the taper between the discrete values obtained by such calculation.

Preferably the channels are semicircular in cross section with a decreasing radius taper and the incident electromagnetic waves are in the TES mode of wave propagation.

Preferably the predetermined law with which the taper from the input port of said one channel and from the output port of said other channel is dimensioned is a cos law.

According to a feature of the invention a band branching network includes a bifurcation, for transferring energy at one end thereof from a circularly sectioned waveguide to two semicircularly sectioned waveguides at the other end thereof, which is coupled to a centre excited coupler, for transferring energy below a predetermined frequency from two semicircularly sectioned waveguides to an axially, centrally disposed waveguide having a further different crosssection, and which in turn is coupled to a reflection filter in accordance with this invention; the combination 7 being capable of operating such that an incident wave possessing the TE mode in circularly sectioned waveguide is transferred to the TEfi mode in each of the two semicircularly sectioned waveguides of the bifurcation, v

the TEfi mode waves in each said semicircularly sectioned waveguide being in phase opposition so that flat sides of the semicircular waveguides and is capable of supporting TE'E, mode waves.

A diplexer, known per se, may be connected to the output of the waveguide having the different cross section to provide further band branching.

The invention will now be described, by way of example, with reference to the accompanying drawings in which:

FIG. 1 shows an axial cross section of a TE mode reflection filter in accordance with this invention, and

FIG. 2 shows a broadband band branching network for converting a predetermined band of energy from the TEfi mode to the TEE mode and which utilises a reflection filter as shown in FIG. 1.

In the figures like numbers denote like parts.

Referring to FIG. 1 a microwave reflection filter shown therein comprises two waveguide channels, 1, 2 each of which has an input port 3, 4 respectively and an output port 5, 6 respectively. The channels 1, 2 are semicircularly cross sectioned and provided with tapers that reduce the waveguide dimensions from the input ports 3, 4 and from the output ports 5, 6 such that the radius of the semicircular cross section reduces towards a region at the centre of the reflection filter. The tapers are arranged to reduce the dimensions of the filter down to those dimensions corresponding to a predetermined upper cut-off frequency, signals below which frequency are reflected and above which frequency are propagated substantially unaffected therethrough.

Taper 7, from the input port 3 and from the output port 6, narrows the waveguide dimensions towards the centre region of the reflection filter and is shaped and dimensioned in accordance with a cos law. Taper 8, from the input port 4 and from the output port 5 narrows the dimensions towards the centre region of the reflection filter and is shaped and dimensioned such that the distance from input port 4 to the point of cutoff for any frequency in the reflected band is substantially a quarter wavelength, at that frequency, greater than the distance from the input port 3 to the corresponding cut-off point in channel I. This is achieved by selecting the cut-off frequency and dimensioning taper 7 so that the required band of frequencies are reflected thereby. Taper 8 is then dimensioned by calculating the required additional distances for a number of discrete frequencies and interpolating the shape of the taper between the discrete values obtained by such calculation. Taper 8 is therefore arranged such that signals of the lowest reflected frequency travel a distance of half a wavelength of the lowest reflected frequency further than the same frequency waves in channel 1 before emerging from ports 4 and 3 respectively. Similarly signals of the highest reflected frequency travel a distance of half the wavelength of the highest reflected frequency further than signals in channel 1 before emerging from ports 4 and 3, with appropriate differences in distances being provided for intermediate frequencies. The input ports 3, 4 and output ports 5, 6 are connected to flanges 9, 10 respectively.

In the operation of a practical embodiment of a reflection filter in accordance with the present invention, electromagnetic wave of the TEE mode in semicircularly sectioned waveguide and which have a frequency range 30 to 90 GHz are provided at input ports 3, 4 but with the phase of the energy entering port 3 differing from the energy entering port 4 by 1r radians. The predetermined cut-off frequency of channels 1, 2 is arranged to be 40 GHz. Thus energy above 40 GHz passes through the reflection filter and out of the ports 5, 6 substantially unaffected by the tapers 7, 8. The tapers at the input and output ports of the channels 1, 2 are conversely shaped and dimensioned so that the 11 radian phase relationship between the energies emerging from ports 5 and 6 is restored. Energy below 40 GI-Iz is reflected by the tapers 7, 8 to pass back through the input ports 3, 4. Because the tapers 7, 8 are shaped and dimensioned such that the phase difference between the reflected waves leaving channels 1, 2 over the reflected frequency range of 30 to 40 GHz is 'n' radians, the reflected energy from channels 1, 2 is now inphase.

Thus, because the tapers 7, 8 are differently shaped and dimensioned by a factor of a quarter wavelength at each discrete reflected frequency it is possible to phase match the reflected energy over a broad band width whereas with the previously described prior art reflection filter only reflected energy of a very narrow bandwidth could be phase matched.

Although the invention has been described with reference to taper 7 having a cos law it is to be understood that other laws such as cos, as well as at least two linear tapers having different slopes, are also believed to be suitable by the applicants. However these other law tapers have not yet been tried in a practical embodiment. In this respect the shape of the taper 7 is determined by two main factors (i) the avoidance of the generation of unwanted modes such as TE and higher order modes, and (ii) the desire to keep the length of the taper as short as possible. It is also to be understood that the reflection filter may have any shaped crosssection suitable for propagating the TE mode.

Referring to FIG. 2 a broadband, band branching network comprises a circular to semicircular bifurcation 11 connected to a centre excited semicircular to rectangular coupler 12 which in turn is connected to a reflection filter 13, having channels with a semicircular cross section, and which is in accordance with the present invention.

The bifurcation 11 has a circularly sectioned input port 14 and a flat sided wedge portion 15 forming two semicircularly sectioned output ports 16 which are connected to two semicircular waveguides 17,18 of the coupler 12. Axially and centrally disposed between the flat sides of the semicircular waveguides 17, 18 of the centre excited coupler is a rectangular wave guide 19 having its narrow walls coupled to the semicircular waveguides l7, 18 by longitudinally disposed axial coupling slots 20, only three of which are shown in each narrow wall for clarity. The coupling slots 20 are shaped and dimensioned in accordance with known principles to resonate at a frequency approximately 25% higher than the highest frequency to be propagated by the semicircular waveguides l7, l8 and are equally spaced apart by Xg/Z, where Ag is the wavelength of a frequency approximately 15% above the highest frequency to be propagated by waveguides 17, 18. The rectangular waveguide 19 is provided with ports 21, 22, port 21 being the output port and port 22' being terminated in a matched load (not shown). The semicircular waveguides 17, 18 are connected by flange 9 to the semicircular input ports 3, 4 of the reflection filter 13.

In operation energy in the frequency range 30 to 90 Gl-lz in the TE mode in circularly sectioned waveguide is applied to input port 14. The bifurcation 11 divides the TE mode energy substantially equally into the TE mode, operating in semicircularly sectioned waveguide, without substantial reflection or disturbance. As will be apparent to those skilled in the art, the magnetic fields of the semicircular axial electromagnetic waves entering waveguides 17, 18 and propagated therealong are phase displaced by w radians at the flat side of the semicircularly sectioned waveguides 17, 18 adjacent rectangular waveguide 19. Because the electromagnetic waves are phase displaced there is substantially no coupling between waveguides 17, 18 and waveguide 19, so that substantially all the energy entering port 14 is propagated into the input ports 3, 4 of the reflection filter 13.

Energy above the cut-off frequency (40 GHZ) of the reflection filter 13 passes substantially unaffected through the filter l3 and out of the ports 5, 6. As previously described, however, energy below 40 GHz is reflected by the tapers 7,8 back into the semicircular waveguides 17, 18 with a phase matched condition. Now that such an in-phase condition exists the reflected TE mode energy in the frequency range 30 to 40 GHz is coupled by the slots 20 into the rectangular waveguide 19 operating in the TEE, mode and thence out of port 21 for utilisation.

In a further embodiment of the invention, energy bands coupled out of the rectangularly sectioned waveguide are further divided by a diplexer, known per se connected to the output port 21.

The band branching network described above is a reciprocal device and as such can also be employed as a band combining network.

Thus a band branching network, operable over broad bandwidths, as defined, may be manufactured using the reflection filter of the present invention.

I claim:

1. A microwave reflection filter including two waveguide channels, an input port and an output port for each said channel, tapers between said input and output ports such that the channel dimension reduces from said input ports to a region between said ports, down to dimensions corresponding to a predetermined upper cut-off frequency such that signals with frequencies above said cut-off frequency pass through the output ports and signals with frequencies below said upper cut-off frequency are reflected through the input ports, the taper from the input port of one of said channels being dimensioned in accordance with a predetermined law and the taper from the input port of said other channel being dimensioned such that the distance from the input port to the point of cut-off for any frequency in the reflected band is substantially a quarter waveguide wavelength, at that frequency, greater than the distance from the input port of said one channel to the corresponding cut-off point in said one channel.

2. A microwave reflection filter as claimed in claim 1 wherein said predetermined law is of any desired suitable form and the shape of the other taper is obtained by calculating said distance for a number of discrete frequencies and interpolating the shape of the taper between the discrete values obtained by such calculation.

3. A microwave reflection filter as claimed in claim 1 wherein the channels are semicircular in crosssection with a decreasing radius taper and the incident electromagnetic waves are in the TE? mode of wave propagation.

4. A band branching network including a bifurcation, for transferring energy at one end thereof from a circularly sectioned waveguide to two semicircularly sectioned waveguides at the other end thereof, which is coupled to a centre excited coupler, for transferring energy below a predetermined frequency from two semicircularly section waveguides to an axially, centrally disposed waveguide having a further different crosssection, and which in turn is coupled to a reflection filter as claimed in claim l the combination being capable of operating such that an incident wave possessing the TE mode in circularly sectioned waveguide is transferred to the TB? mode in each of the two semicircularly sectioned waveguides of the bifurcation, the TEfi mode waves in each said semicircularly sectioned waveguide being in phase opposition so that they pass through the centre excited coupler to the reflection filter, whereupon energy above said predetermined frequency passes through the reflection filter substantially unaffected thereby and energy below said predetermined frequency is reflected by the reflection filter so as to provide an in-phase relationship between the reflected TEfi mode waves which are then coupled into the waveguide having the further different crosssection.

5. A band branching network as claimed in claim 4 wherein said further, different cross-section is rectangular with the narrow walls thereof connected to the flat sides of the semicircular waveguides and is capable of supporting TEE, mode waves.

6. A band branching network as claimed in claim 4 wherein a diplexer known per se is connected to the output of the waveguide having the different crosssection to provide further band branching.

7. A microwave reflection filter as claimed in claim 1 wherein the waveguide channels include tapers between said region and the output ports, the taper in said other channel from the output port to said region being in accordance with said predetermined law and the taper in said one channel from the output port to said region having the same dimensions as the taper in said other channel from the input port to said region.

8. A microwave reflection filter as claimed in claim 7 wherein the predetermined law with which the taper from the input port of said one channel and from the output port of said other channel is dimensioned is a 

1. A microwave reflection filter including two waveguide channels, an input port and an output port for each said channel, tapers between said input and output ports such that the channel dimension reduces from said input ports to a region between said ports, down to dimensions corresponding to a predetermined upper cut-off frequency such that signals with frequencies above said cut-off frequency pass through the output ports and signals with frequencies below said upper cut-off frequency are reflected through the input ports, the taper from the input port of one of said channels being dimensioned in accordance with a predetermined law and the taper from the input port of said other channel being dimensioned such that the distance from the input port to the point of cut-off for any frequency in the reflected band is substantially a quarter waveguide wavelength, at that frequency, greater than the distance from the input port of said one channel to the corresponding cut-off point in said one channel.
 2. A microwave reflection filter as claimed in claim 1 wherein said predetermined law is of any desired suitable form and the shape of the other taper is obtained by calculating said distance for a number of discrete frequencies and interpolating the shape of the taper between the discrete values obtained by such calculation.
 3. A microwave reflection filter as claimed in claim 1 wherein the channels are semicircular in cross-section with a decreasing radius taper and the incident electromagnetic waves are in the TE01 mode of wave propagation.
 4. A band branching network including a bifurcation, for transferring energy at one end thereof from a circularly sectioned waveguide to two semicircularly sectioned waveguides at the other end thereof, which is coupled to a centre excited coupler, for transferring energy below a predetermined frequency from two semicircularly section waveguides to an axially, centrally disposed waveguide having a further different cross-section, and which in turn is coupled to a reflection filter as claimed in claim 1 the combination being capable of operating such that an incident wave possessing the TE01* mode in circularly sectioned waveguide is transferred to the TE01 mode in each of the two semicircularly sectioned waveguides of the bifurcation, the TE01 mode waves in each said semicircularly sectioned waveguide being in phase opposition so that they pass through the centre excited coupler to the reflection filter, whereupon energy above said predetermined frequency passes through the reflection filter substantially unaffected thereby and energy below said predetermined frequency is reflected by the reflection filter so as to provide an in-phase relationship between the reflected TE01 mode waves which are then coupled into the waveguide having the further different cross-section.
 5. A band bRanching network as claimed in claim 4 wherein said further, different cross-section is rectangular with the narrow walls thereof connected to the flat sides of the semicircular waveguides and is capable of supporting TE10 mode waves.
 6. A band branching network as claimed in claim 4 wherein a diplexer known per se is connected to the output of the waveguide having the different cross-section to provide further band branching.
 7. A microwave reflection filter as claimed in claim 1 wherein the waveguide channels include tapers between said region and the output ports, the taper in said other channel from the output port to said region being in accordance with said predetermined law and the taper in said one channel from the output port to said region having the same dimensions as the taper in said other channel from the input port to said region.
 8. A microwave reflection filter as claimed in claim 7 wherein the predetermined law with which the taper from the input port of said one channel and from the output port of said other channel is dimensioned is a cos2 law. 